fix #5 camera video preview with settling and diagnostics

- Wait until the picker movie's file size stops changing before
  staging it, so a movie whose final bytes are still being flushed is
  never copied truncated (which produced drafts without a thumbnail
  and unplayable previews).
- Log the whole camera video path (pickup size, staging result,
  analyze outcome, thumbnail decode) under the video-preview os_log
  category for fast on-device diagnosis.
- Cover the video analysis pipeline with a simulator test that
  generates an HEVC QuickTime movie and asserts duration plus JPEG
  thumbnail, and guard the diagnostics in Scripts/verify.sh.

Scripts/verify.sh

@@ -83,6 +83,7 @@ required=(
   Tests/ArchiveStoreTests.swift
   Tests/SASLprepTests.swift
   Tests/SaslFailureMessageTests.swift
+  Tests/MediaPreviewProcessorVideoTests.swift
   Sources/Shared/XMPP/SASLprep.swift
   Sources/Shared/XMPP/LumaSaslFailureModule.swift
   Sources/Shared/XMPP/SaslFailureMessage.swift
@@ -348,6 +349,10 @@ grep -q 'moveItem(at: sourceURL' Sources/Shared/UI/SystemPhotoCameraView.swift |
   echo "The picker movie must be claimed synchronously before dismissal cleanup"
   exit 1
 }
+grep -q 'video-preview' Sources/Shared/UI/ChatView.swift || {
+  echo "The camera video path must log diagnostics"
+  exit 1
+}
 grep -q 'ArchiveSyncCursorPolicy.requestPosition' Sources/Shared/XMPP/XMPPService.swift || {
   echo "MAM reconnects must prefer the durable archive UID over timestamps"
   exit 1